1. Field of the Invention
The present invention relates radiation therapy, and more specifically it relates to a method for condensing the photon energy and angular distributions obtained from Monte Carlo simulations of medical accelerators.
2. Description of Related Art
Monte Carlo methods are valuable for radiation dose calculations because of their ability to predict dose accurately for the entire range of conditions encountered in radiation treatment of cancer. They can correctly characterize beams of electrons and photons emerging from a linear accelerator, as well as photons, electrons and alpha particles emerging from brachytherapy sources. They can further accurately simulate the transport of particles through beam shaping devices or, for brachytherapy, source shielding devices. The transport of the resulting particles through the patient may then similarly be simulated, taking into account surface irregularities and the internal structure of the patient, including any metallic implants and prostheses, to determine the dose deposited in the patient. This disclosure provides photon treatments with linear accelerators.
While Monte Carlo methods are the most accurate means of predicting radiation dose, the use of generalized Monte Carlo software packages for routine clinical applications is presently impractical due partly to computer constraints, in particular, the running time needed to obtain a converged calculation. The lengthy running time of generalized Monte Carlo software packages warrants the development of algorithms optimized specifically for the problem at hand. The problem may be divided into three steps: (1) Determining the characteristics of the photon radiation emanating from the accelerator head; (2) propagating this radiation down to the patient, through beam modifiers (if any) and through the air above the patient; and (3), the actual transport within the patient of the original radiation, as well as of any radiation produced or scattered between the accelerator head and the patient.
It is an object of the present invention to provide, inter alia, a method for condensing the photon energy and angular distributions obtained from Monte Carlo simulations of medical accelerators. This method represents the output as a series of correlated histograms and as such is well-suited for inclusion as the photon-source package for Monte Carlo codes used to determine the dose distributions in photon teletherapy.
The method accounts for the isocenter-plane variations of the photon energy spectral distributions with increasing distance from the beam central axis for radiation produced in the bremsstrahlung target as well as for radiation scattered by the various treatment machine components within the accelerator head. Comparison of the isocenter energy fluence computed by this algorithm with that of the underlying full-physics Monte Carlo photon phase-space indicates that energy fluence errors are less than 1% of the maximum energy fluence for a range of open-field sizes. Comparison of jaw-edge penumbrae shows that the angular distributions of the photons are accurately reproduced. The Monte Carlo sampling efficiency (the fraction of generated photons which clear the collimator jaws) of the algorithm is approximately 83 % for an open 10xc3x9710 field, rising to approximately 96% for an open 40xc3x9740 field. Data file sizes for a typical medical accelerator, at a given energy, are approximately 150 kB, compared to the 1 GB size of the underlying full-physics phase-space file.